Fuel tanks on vehicles and often tanks used to hold reagents for chemical processes must have openings through which liquids can be made to flow into such tanks. The art of filler-cap assemblies is well developed. A simple example is the ordinary filler-cap used for the gasoline tank of an automobile.
In the case of an automobile the consequences of leaving the cap off after filling or the cap being somehow mechanically dislodged are usually not great. In the case of for example the oil tank associated with a jet engine however, leaving the filler-cap off could result eventually in engine failure. In many processes hazardous chemical compounds could be released from a tank as a result of the failure to properly seal the tank.
It is of course possible to provide electro-mechanical devices to signal the fact that a closure device is not properly positioned or to provide a signal which inhibits the start of a process. There are times however, when this approach is not sufficiently reliable, is too costly or when general application considerations cause it to be undesirable.